1. Field of the Invention
The present invention relates to a field effect transistor capable of easily adjusting switching speed thereof, and more particularly, to a vertical-type insulated-gate field effect transistor provided with a structure for adjusting a switching speed thereof.
2. Description of the Related Art
Such a transistor in the prior art is illustrated in FIGS. 1(a) and 1(b). A manufacturing method therefor is described hereinafter. First, on an n.sup.+ -type silicon substrate 1 is grown an n-type epitaxial layer 2 on which a gate oxide film 3 is formed. Then, at a desired place on the gate oxide film 3, a polysilicon layer is formed as a gate electrode 4. Using this gate electrode 4 as a mask, a base region 9 is formed by an ion implantation of p-type impurities. Subsequently, an aluminium layer (not shown) is selectively formed on the center portion of the base region 9, and then using the gate electrode 4 and the aluminium layer as masks, a source region 10 is formed by an ion implantation. After teh aluminium layer is removed, an oxide film 12 is deposited, and holes are opened at the appropriate places of the oxide film 12 and a source electrode 14 and a gate bonding pad 7 are formed. A drain electrode 15 is formed on the back surface of the silicon substrate 1. Referring to FIGS. 2 and 3, in the case of this conventional transistor 20, there is capacitance C.sub.GS and C.sub.GD due to the oxide films 12 and 3 for isolation between the source and the gate and between the gate and the drain, respectively. The switching speed depends on the input capacitance C.sub.GS and C.sub.GD and the capacitance is a function of electrode area dielectric constant and distance between electrodes and hence the switching speed is affected greatly or limited by the structure of the gate electrode. As the drawback, therefore, control of the switching speed by modifying the gate electrode structure inevitably affects other characteristics.